Compounds useful in the prevention and/or treatment of hypertension, congestive heart failure, arrhythmia, migraine, vasospastic disorders, and asthma are represented by the formula IMAGE (I) wherein: R, R1, R2, R3, and R4 are independently hydrogen or lower alkyl of one to four carbon atoms; and R5 and R6 are independently hydrogen, lower alkyl of one to four carbon atoms or lower alkoxy of one to four carbon atoms; and the pharmaceutically acceptable acid addition salts thereof.
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